A positive motion belt with elastic teeth (hereinafter called the belt) of conventional type 1 has teeth 2 of all-addendum form, lacking part of the tooth tip and tooth root. The tooth surface 3 is straight, having a pressure angle .alpha..sub.o. Reference numeral 4 designates the root fillet curve where the tooth root joins the bottom land 5. At the back of the bottom land 5 is laid a load carrying member 6, so that the pitch line 7 runs through the middle of said load carrying member 6. The tooth surface 3, bottom land 5 and top land 8 of the tooth 2 are covered, as required, with canvas or other material. On the inside of the tooth 2 and the back 9 of the belt 1 are filled rubber or rubber-like elastic material.
When the belt 1 is placed on wheels 40 and 40' as illustrated in FIG. 2, individual pitch circles 45 and 45' of the wheels 40 and 40' must overlap with the pitch line 7 of the belt 1. Therefore, the pitch circles 45 and 45' of the wheels 40 and 40' exist outside their addendum circles 44 and 44'. Consequently, their teeth 41 and 41' assume the all-addendum form. The belt 1 is manufactured by winding its material around an external gear 11 having teeth 12 shown in FIG. 3. The tooth space 13 and the tooth surface 14 of the external gear 11 form a tooth 2 and a tooth surface 3 respectively, of the belt. To make a belt having a straight-sided teeth, the tooth surface 14 of the external gear 11 must be straight-lined. The external gear 11 having such straight-sided teeth can be cut by the so-called single cutting machine. However, this tooth cutting method has such shortcomings as follows:
I. Since tooth cutting must be effected while driving one tooth after another, tooth pitch error is liable to be introduced.
II. To permit the tip fillet curve 15 of the external gear 11, that forms the root fillet curve 4 of the belt 1, to smoothly join the outside diameter, it is necessary to use cutting machinery, workmanship and material of great accuracy for the manufacture of said external gear, which necessitates much time and labor.
III. A gear cutting operation according to this method is inefficient.
The above-described shortcomings can be obviated by generating the external gear 11 by topping, utilizing a rack-type tool. However, the form of the tooth of the gear generated by the rack-type tool with straight-sided teeth is not straight-sided but involute. Therefore, the tooth form of the rack-type tool must be designed with a special curve in order to obtain a gear with straight-sided teeth by the generating process. In addition, the tooth form of the tool must be changed when the number of gear teeth changes. That is, a different rack-type tool is required for every different number of teeth of the external gear. This means that a great number of costly rack-type tools must be prepared. The manufacture, maintenance and keeping of such tools will cost a great deal, which, as a consequence, makes the generation of an external gear with straight-sided teeth by the use of the rack-type tool very expensive.
The present invention eliminates the aforesaid shortcomings, as described hereafter with reference to the accompanying drawings.